Many satellite mobile communication applications require that the direction of maximum sensitivity or gain of a receiving antenna be adjusted; i.e., that the receiving antenna be directed towards the satellite and track the satellite while the vehicle is moving and turning.
Typically, in the continental United States television satellites may be between 30° and 60° above the horizon. In mobile satellite television applications, operating in a 12 GHz range, standard dish antennas may be mounted on the vehicle and mechanically rotated to the appropriate azimuth and tilted to the appropriate elevation to track the satellite.
While such systems may provide adequate signal acquisition and tracking, the antenna, tracking mechanism and protective dome cover may present a profile on the order of 15 inches high and 30 inches or more in diameter. This size profile may be acceptable on marine vehicles, commercial vehicles and large recreational vehicles, such as motor homes. However, for applications where a lower profile is desirable, a special low profile dish antenna, or a planar antenna element, or array of elements may be preferred. However, low profile dish antennas may only decrease overall height by two to four inches. Planar antennas suffer in that maximum gain may be orthogonal to the plane of the antenna, thus not optimally directed at a satellite, which may be 60° from that direction.
In a planar phased array antenna, a stationary array of antenna elements may be employed. The array elements may be produced inexpensively by conventional integrated circuit manufacturing techniques, e.g., photolithography, on a continuous dielectric substrate, and may be referred to as microstrip antennas. The direction of spatial gain or sensitivity of the antenna can be changed by adjusting the relative phase of the signals received from the antenna elements. However, gain may vary as the cosine of the angle from the direction of maximum gain, typically orthogonal to the plane of the array; and this may result in inadequate gain at typical satellite elevations. Attempts have been made to change the direction of maximum gain by arranging microstrip elements in a Yagi configuration. For example, see U.S. Pat. No. 4,370,657, “Electrically end coupled parasitic microstrip antennas” to Kaloi; U.S. Pat. No. 5,008,681, “Microstrip antenna with parasitic elements” to Cavallaro, et al.; and U.S. Pat. No. 5,220,335, “Planar microstrip Yagi antenna array” to Huang.
In another configuration described in “MSAT Vehicular Antennas with Self Scanning Array Elements,” L. Shafai, Proceedings of the Second International Mobile Satellite Conference, Ottawa, 1990, and referred to herein as a dual mode patch antenna, an element tuned to a fundamental mode can be stacked above an element tuned to a second mode. To date, these attempts have had limited success as mobile communications antenna and have proved impractical as phased array antenna in general.